Consistent fractionation of¹³C in nature and in the laboratory: Growth-rate effects in some haptophyte algae

The carbon isotopic fractionation accompanying formation of biomass by alkenone-producing algae in natural marine environments varies systematically with the concentration of dissolved phosphate. Specifically, if the fractionation is expressed by ε≈ δ_e - δ_p, where δ_e, and δ_p are the δ¹³C values for dissolved CO₂ and for algal biomass (determined by isotopic analysis of C₃₇ alkadienones), respectively, and if C_e is the concentration of dissolved CO₂, μmol kg^-1, then b = 38 +160*[PO₄], where [PO₄] is the concentration of dissolved phosphate, μM, and b = (25 - ε_P)C_ε. The correlation found between b and [PO₄] is due to effects linking nutrient levels to growth rates and cellular carbon budgets for alkenone-containing algae, most likely by trace-metal limitations on algal growth. The relationship reported here is characteristic of 39 samples (r² = 0.95) from the Santa Monica Basin (six different times during the annual cycle), the equatorial Pacific (boreal spring and fall cruises as well as during an iron-enrichment experiment), and the Peru upwelling zone. Points representative of samples from the Sargasso Sea ([PO₄] ≤ 0.1 μM) fall above the b = f[PO₄] line. Analysis of correlations expected between μ(growth rate), ε_P, and C_e shows that, for our entire data set, most variations in ε_p result from variations in μ rather than C_e. Accordingly, before concentrations of dissolved CO₂ can be estimated from isotopic fractionations, some means of accounting for variations in growth rate must be found, perhaps by drawing on relationships between [PO₄] and Cd/Ca ratios in shells of planktonic foraminifera.